Telephone system



p 1935. H. HOVLAND I 2,014,232

TELEPHONE SYSTEM Filed July 20, 1953 2 Shets-Sheet 1 c5-5- 5 IN TERRUP TENS INVENTOR H. H0 VLAND ATTOR EV Sept. 10, 1935; I H. HOVLAND 2,014,232

' TELEPHONE SYSTEM" Filed July 20, 1953 2 Sheets-Sheet 2 IIHI INVENTOR y H. HOVLANDY- Patented Sept. 10, 1935 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application July 20, 1933, Serial No. 681,268

16 Claims.

.. tral office switches for a reasonable period of time following release by the calling party; to prevent the equipment from being held out of service indefinitely due to trouble; to prevent the equipment that has already been used for one call and has not yet been fully released from inter- ;fering with the operation of the equipment that is being used in the establishment of a second connection, and to otherwise improve systems .of this character.

' It is well known to have the calling subscriber control the release of the entire connection, including the selector andconnector switches, when he replaces his receiver on the switchhook. In such cases, however, if the called subscriber does not hang up his receiver immediately, his line is extended by a finder or equivalent switch the .same as it would if he were originating a call. This takes out of service equipment that might otherwise .be used by other subscribers.

It has also been proposed heretoforeparticu- ,larly in systems employing switches of the step- .by-step type, to have the calling subscriber release all switches except the connector switch, this switch remaining in its set position until the called subscriber retires. With this plan the connector switch. is held unduly long in casethe called subscriber does not hang up his receiver. Moreover, a trouble condition in the .line may hold the connector indefinitely.

According to the present invention, these difficulties are overcome, in systems .of the characterabove described, by the provision of means whereby-the calling subscriber on replacing his receiver causes the removal of the .holdingpotential in the connector switch to permit the immediate releaseof all switches preceding the connector, but without causing the release of the connector switch. After a brief interval the holding potential is replaced toprevent the connector switch frombeing seized by. otherselectors while it is being held by the called subscriber. After a still further interval, the connector switch is automatically released althoughv the called subscribermay still have his receiver oif the hook. This interval between the time when local and toll connections.

the calling subscriber hangs up his receiverand the release of the connector switch is sufficiently long to enable the called subscriber to hang up his receiver.

Another feature of the invention is anarrange- 5 ment in which the circuit that feeds current from M the connector switch in the direction of the calling subscribers line is opened during the interval that the connector switch is unguardedlso that if the connector is seized during this inter-l0 Val, current will not flow, from theconnector over the trunk in the direction of the calling subscribersline that is seeking to establish a new connection. The reason for this is toprevent the operation of a polarized supervisory relay due to;l 5 the fact that the battery and ground leads in the connector are reversed since the called subscriber has already answered.

Other features of the invention will appear from the following detailed description when con- 20 sidered in connection with the accompanying drawings.

Fig. l of the drawings illustrates in schematic form a. calling subscribers line Hill, a linefinder switch F and first and second selector switc'hes 25 Sl and S--2. The drawings also show the. de-' tails of a connector switch C and a calledsubscribers line 1M. Fig. 2 shows the details of a connector switch C-| involving a modification of the invention. 3,0

The finder, selector and connector switches shown in the drawings are of the well known twomotion step-by-step type; but the invention is, of course, applicable to systems which use other types of switches and employ different trunking,.l. arrangements.

A detailed description will now be given of the circuits shown in Fig. 1, assuming that the subscriber of the local line I00 wishes to converse with the subscriber of line IBI. When the calling fl subscriber lifts his receiver, the line finder F operates. in the well-known manner to seize the calling line. Thereupon the calling subscriber operates his dial to set the selector switches S-'-l and S2. Selector switch S2 in its huntingA- movement seizes the terminals I02, I03, and 104 of an idle connector C. It may be noted at this point that the connector switch Cserves for both For example' when a toll call is made this connector is seized by, a toll'selector (not shown) through the tenninals" I05, I06, 101, and ma.

Seizure of the connector switch by theselector S2 results in the closure of a circuit .for the impulse relay I09 by means of which the calling sub-,;-.

scriber controls the vertical and rotary movements of the connector. This circuit can be traced from battery through the right-hand winding of relay I09, conductor H0, armature and contact of relay II I, conductor II2, upper normal contact of relay I I3, thence over the ring side of the line and returning over the tip side through the inner upper normal contact of relay II3 to ground through the left-hand winding of relay I09. Relay I09 operates the slow-to-release relay II4 which applies ground at its outermost contact over conductor II5 to the multiple test terminal I02 in the local selector banks and also through the left contact of relay H8 and the lower contact of relay II3 to the test terminals I06 in the toll selector banks. Relay II4 also alters the loop circuit just traced for the relay I09 so that the relay I09 is now independent of the relay I I I. This will be seen by tracing the circuit from battery through the right winding of relay I09, through the middle contact of relay 4, thence to the subscribers line and thence over the subscribers line and returning as above explained through the left winding of relay I09.

The calling subscriber now manipulates his dial to transmit the first series of impulses, and relay I09, upon releasing in response to the first opening of the circuit, completes the stepping circuit for the vertical magnet from ground through the armature and back contact of relay I09, front contact of relay II l, conductor II1, closed springs H8 and I20, through the winding of slow-release relay I2 I, vertical magnet I22, to battery. Relay I2I operates and remains energized during the series of impulses and magnet I22 steps the brushes I23, I24, and I25 one vertical step. On the first vertical step of the switch the off-normal contacts are shifted so that the pulsing circuit over conductor I I1 now extends through springs H8 and H9, lower armature and front contact and winding of relay I2 I to the vertical magnet. The magnet I22 responds to the succeeding pulses in the first series and advances the brushes of the connector to the desired level of lines.

Following the first series of pulses the circuit of relay I2I is maintained open for a sufficient interval to permit its release. Relay I2I upon releasing shifts the pulsing circuit from the vertical magnet I22 to the rotary magnet I20. When,

therefore, the first impulse of the next series is 3 into engagement with the first subscribers line in the chosen level. Relay I23 alters the stepping circuit so as to render it independent of the test relay I21. Therefore, when the second and all succeeding pulses of the second series are received, they are delivered to the rotary magnet over a circuit including the conductor II1, contacts H8 and II9, back contact of relay I2I,

thence through the left front contact and armature of relay I28, back contact of relay III, to

. the magnet I20. At the end of the second series of pulses the brushes of the connector come to rest on the terminals of the called subscribers line IOI.

The called line is now tested, and, if found busy, the test relay I21 operates in a circuit from battery through its lower winding, inner upper armature and back contact of relay III, through the right front contact of relay I28, which relay is 5 holding its armature attracted for a brief interval following the delivery of the last pulse, to the test wiper I23 and the busy test terminal of the called line. Relay I21 operates and in so doing prevents the application of ringing current 10 to the called line and, furthermore, applies a busy-tone source to the calling subscribers line to notify him that the called line is engaged. The circuit for the busy-tone current may be traced from the busy-tone coil I29, right-hand back contact of relay I30, front contact of relay I21, thence to the upper talking conductor and to the calling subscribers station. The calling subscriber hearing the busy tone replaces his receiver on the hook, thus causing the release of the switches in a manner to be explained more in detail hereinafter.

Assume, however, that the called subscribers line is idle at the time. In this case the test relay I21 fails to operate at the end of the rotary 25 movement of the switch, and, when relay I28 restores its armatures, a circuit is completed for relay II I. This circuit may be traced from battery through the winding of the cut-off relay I3I of the called line, brush I23, right-hand back 3 contact of relay I23, lower winding of relay III, back contact of relay I21, tothe grounded conductor II5. Relay II I operates and locks in a circuit from battery through its upper Winding and through its front contact, conductor I32, oif-normal springs I33 and I34, conductor I35, lower normal contact of relay I I3, left back contact of relay I IE, to the grounded conductor H5. The cut-off relay I3I in. operating disconnects the line relay I36, and relay III at its uppermost armature applies a busy potential to the terminals of the called subscribers line. Relay III also applies ground potential tothe holding conductor I I5 in a circuit from ground through its uppermost armature, right back contact of relay I28, lower winding and contact of relay I I I, to the conductor I I5. Another branch of this circuit extends from the lower winding of relay I I I, through the back contact of relay I21 to the conductor II 5.

A circuit is now prepared by the operation of relay I31 for ringing the called subscribers line. This relay operates in a circuit from battery through its winding, lower back contact of the ringing trip relay I 38, lower normal contact of relay I31, front contact of relay III, conductor I39, and thence to ground through a pick-up interrupter (not shown) as soon as said interrupter reaches its next closing point. Relay I31 in operating looks through its winding and lower front contact, thence over conductor I32, to the grounded conductor I35 as above traced. Relay I31 applies ringing current in a circuit from the ringing source I 40, upper front contact of relay I31, upper winding and uppermost back contact of the slow-operating trip relay I38, front contact of relay II I, brush I24, thence over the loop of the called line and returning through brush I25, front contact of relay III, back contact of relay I38, conductor I4I, to ground. The relay I38, being of a marginal character, does not operate in series with the ringer at the called substation.

When the called party responds by removing his receiver, the resistance of the loop is suffito the grounded .conductor .135.

iciently reduced to permit the trip relay .138 to attract itsarmatures and disconnect theringing of these same relays to-ground through the lower -winding of relay H3. 15'

- from the windings of relay I 9.9, to the calling Relay H3 at its upper front contacts reverses the direction of current subscriber's line. This reversal of current may serve the well-known purpose of operating a polarized relay I42. The reversal of current through the winding of relay I42 may be used to effect any desired supervisory function including indication of the answer of the called subscriber. Since the calling subscribers cut-off relay I43 is in an operated condition, the message register I44 of the calling line is operated in a circuit closed at the front contact of the polarized relay I42.

When relay H3 operates, it shifts the holding circuits of relays H I and I38 to the back contact of the test relay I21. The holding circuit of relay II I now extends from battery through the upper winding and front contact of said relay, and thence as traced over conductors I32 and I35, lower front contact of relay H3, conductor I45, to ground at the back contact of relay I21. The holding circuit of relay I38 may be traced from battery through its lower winding and front contact, back contact of relay I30, and thence to ground as just traced at the back contact of relay I21.

Talking current is supplied to the calling line through the windings of relay I09 and to the called subscribers line, through the windings of relay H3.

It will be assumed that the call'ng subscriber is the first to replace his receiver when the conversation is finished. This results in the release of relay I09. Relay Hi9 completes a circuit, while slow relay H4 is maintaining its armatu-res, over conductor H1, and thence as previously traced through the winding of slow-release'relay I28, I

Y to battery. Relay I28 operates, and, after a brief interval, relay Hd'releases.

Relay II l on releasing removes ground potential from conductor I I5 and also opens the circuit of relay I23. Since relay I28 is now maintaining its armatures retracted fora brief interval, the circuits previously traced from ground through the upper front contact of relay III to the holding conductor H5 are held open momentarily. With all sources of ground potential thus removed from the conductor H5 the selector switches S! and S2 and the finder switch F are permitted to release while the connector remains in its set position. After connectionthus seeking.a'connector'switch might operate and cause a false actuation of. the calling .subscribers message register. Such anoperation is prevented in this system by opening'the connection of the battery source through the;

right winding of relay I69. This opening occurs at the middle armature and contactof relay H4. .If,therefore, a selector S-2seizes the connector C at any time during the unguarded interval,

no current will flow from the battery source;1

through the right winding of relay I09 over the connection, through the polarized relay I42, be-

cause the circuit for such current how is now open at the contact of relay .I Hi.

Should the called subscriber hang up hisreceiver soon after the calling subscriber, hecauses the release of relay H3, which in turn releases relays I II and I38. The release magnet .I41:is now energized in a circuit through off-normal springs M8 and M9, lower back contact of relay.

HI, conductor I55 to ground through the back contacts of relays IM and we. The magnet I41 restores the connector switch to normal and deenergizes upon the opening of the off-normal springs I48 and I59.

It Will .next be assumed that the called subscriber delays the replacement of his receiver for a considerable period after the calling subscriber hangs up his receiver. To prevent the connector switch C from beingheld indefinitely by the; -closure of the called subscribers loop, which ground through the back contacts of relays I03 and IM, conductor I50, lower front contact'of relay III, upper winding of relay I21, to the front contact of the common timing relay I52. As soon thereafter as relay I52 is operated by the interrupter I55, relay I21 energizesand locks ing, front contact of relay III, lower'front contact. of relay I21, conductor H5, inner lower front contact and lower winding of relay III, right back contact of relay I28, to ground at; the .uppermost armature of. relay III. Relay I21 at its make-before-break contact transfers the conductor M5 from the ground connection at the normal contact of relay I21 to-the ground connection at the armature and back contact of- :4 in .a..circuit from battery through its lowerwindthe timing relay I5I. This means that the holdingcircuits of relays III and I38 hereinbefore traced have now been placed under the control The connector switch C--I shown in Fig. 2 ls' similar in many respects to the one. shown in Fig. 1. In this modification of the invention when the calling line is extended to the connector switch as previously described over a se-,

[.60 is energized by the interrupter I55 lector switch 8-2, the impulse relay200 oper ates in a circuit from battery'and ground through its right and left windings, thenceover contacts .ofthe relay 20I, to the callingsubscribers.line. :Relay @200 eoperates .the..slow,-;release:relay .1202;

and, when the first series of impulses are sent, the vertical stepping magnet 203 responds to advance the brushes to the proper level of lines. On the next series of impulses the rotary stepping magnet 204 is actuated in series with the slow relay 205, and the brushes are stepped to the terminals of the called subscribers line.

If the called line is busy, relay 206 operates following the last pulse in the series from battery through its winding, contact of relay 201, lower front contact of relay 205, to ground on the test brush 208. When relay 205 releases, a circuit is closed from battery through the winding of relay Z09, contact of the rotary magnet 204, and thence through contacts of relays 209, 206, 205 and 2l0, to the grounded test brush 2| Relay 209 operates, and, if the line on which the connector has been positioned is the first line in a group of trunks to a private branch exchange, the magnet 204 steps the brushes to the next lineand releases relay 209. This stepping action continues until an idle line is found or until the last line in the group is reached in the Well-known manner. It should be explained that the test terminals 2|2 and 2 I3 of direct lines are not connected together, whereas those for private branch exchange groups are connected together in all cases except for the last trunk in the group.

Assuming that the called line is a direct line, the relay 206 operates as explained, but relay 209 fails to operate due to the absence of ground potential on the test terminal 2 l2 of the busy line. Relay 2|0 now operates in series with relay 209 in a circuit connected to the grounded test terminal 2| 3. Relay 2"! being of high resistance does not permit the relay 209 to operate. Relay 2|0 locks in a circuit traceable from battery through the winding of relay 209, contact of magnet 204, contact of relay 209, contacts of relays 206 and 205, winding and right front contact of relay 2l0, to the grounded conductor 2| I. The busy-tone source 2 I4 is now connected to the calling line through the contacts of relay 2| 0.

If the called subscribers line is idle, relay 206 fails to operate when the test is made, and, upon the release of relay 205, a circuit is closed from battery through the cut-off relay of the called line over terminal 2|3, thence over .a circuit traceable through the lower winding of relay 201, to ground at the outer armature of relay 202. Relay 201 operates and locks to the grounded conductor 2| I. The tip and ring conductors are now extended through to the called subscribers line, and ringing current is applied from the generator 2|5. When the called subscriber answers, the tripping relay 256 operates and disconnects the ringing source and completes the talking circuit for conversation. The called subscriber's line is included in series with the winding of battery feed relay 20l, and this relay operates, reversing the direction of current over the calling subscribers line. It will be noted that the circuit for the supply of current to the calling line from battery through the right-hand winding of relay 200 is controlled by a contact of relay 202.

The holding conductor 2|1 is grounded at the contact of relay 202 and also by means of a path extending through the middle left contact of re lay 206, lower winding of relay 201, inner back contact of relay 205, to ground at the front contact of relay 201.

When the calling subscriber replaces his receiver following conversation, relay 200 releases,

and a holding circuit is closed for relay 201, traceable from battery through the upper winding and.

contact of said relay, conductor 2| l, upper front contact of relay 20|, left back contact of relay 200, to ground at the back contact of the common timing relay 2|8. Relay 200 also closes an obvious circuit for the reoperation of relay 205.: 5

Relay 205 opens the path from ground at the contact of relay 201, to the holding conductor 2|1. After an interval the slow-release relay 202 retracts its armatures, and the last source of ground is removed from the holding conductor- 2H, and the circuit of relay 205 is opened. The removal of ground from the holding conductor releases the preceding switches, and, after a brief interval, relay 205 again releases and replaces ground potential on conductor 2" to guard the connector switch. During the unguarded interval battery potential is disconnected from the incoming circuit at the contact of relay 202.

Upon the first closure of the common interrupter 23|, relay 2|9 operates over an obvious'zo circuit and locks. Upon the next closure of the interrupter 230, relay 2 l8 operates and opens the holding circuit of relay 201. If the called subscribed has not replaced his receiver by this time, the release of relay 201 opens the circuit of relay'25 L and relay 20| releases to in turn close the following circuit for the release magnet 220: battery through the winding of said magnet, contact of relay 201, oil-normal springs 22| and 222, contacts of relays 20|, 202 and 200, to ground. Magnet 220 restores the switch to normal.

By means of these circuit arrangements it is possible for the calling subscriber to immediately release all of the switches, except the last one, which is held under the control of the called subscriber and will release provided he replaces his receiver within a reasonable time after the calling subscriber. However, in case he delays, the connector is automatically released after a time interval is measured. Moreover, provision is made whereby other subscribers attempting to establish a connection are safeguarded against false operation of the equipment in the event a connector switch is seized during the unguarded interval necessary to enable the release of the switches preceding the connector.

What is claimed is:

1. The combination in a telephone system of a line, a selector switch and a connector switch for establishing a connection to said line, means in said connector for maintaining a holding condition for the established connection, means for removing said holding condition to release the selector switch without releasing said connector switch, and timing means for causing the release of said connector after an interval of time following the release of said selector switch.

2. The combination in a telephone system of a line, a plurality of selective switches operated in succession to establish connection to said line, means in one of said switches for maintaining a holding condition for the established connection, means for removing said holding condition to cause the release of all except said last mentioned switch, and timing means for causing the release of said last mentioned switch after a definite interval following the release of said other switches.

3. In a telephone system, a calling line and a called line, a selector switch and a connector 7 switch for establishing a connection between said lines, means in said connector switch for maintaining a holding condition for the established connection, means under the control of the calling subscriber for removing said holding conlines, a selective switch for establishing connec dition to release the selector switch without releasing said connector switch, means controlled by the called subscriber for releasing said connector switch, and timing means for causing the release of said connector after an interval following the release of said selector switch.

4. In a telephone system, a calling line and a called line, a selector switch and a connector switch for establishing a connection from the calling line to the called line, means for maintaining a potential on the established connection for holding said selector switch, means controlled by the calling subscriber for removing said potential to release the selector switch without releasing said connector, and a timing device for causing the release of the connector switch after a predetermined interval following the release of said selector.

5. The combination in a telephone system of a :calling line and a called line, a selector switch and a connector switch for establishing a connection between said lines, means in said connector for maintaining a potential on the established connection to hold said selector switch, means under the control of the calling subscriber for removing said potential to cause the release of said selector switch without releasing said connector, means for replacing said potential to prevent a reseizure of said connector, means controlled by the called subscriber for releasing said connector switch, and timing means effective if the called subscriber does not replace his receiver to cause the release of said connector switch.

6. The combination in a telephone system of a telephone line, a plurality of selective switches operated in succession to establish a connection to said line, a control circuit extending through said switches, means in a particular one of said switches to maintain a potential on said control circuit to hold the other of said switches against release, means for momentarily removing said potential to cause the release of said other switches without releasing said particular switch, and timing means for causing the release of said last mentioned switch.

'7. In a telephone system, a calling line and a called line, selector switches and a connector switch for setting up a connection between said lines, release control means in said connector responsive to the restoration of the calling subscribers receiver for causing the immediate release of said selectors without releasing said connector, means controlled by the called line for causing the immediate release of the connector after said selectors have released, and time measuring means for efiecting the release of said connector in case the called subscriber does not replace his receiver.

8. In a telephone system, a calling line and a called line, selector switches and a connector switch for setting up a connection between said lines, release control means in said connector responsive to the restoration of the calling subscribers receiver for causing the immediate release of said selectors without releasing said connector, means controlled by the called line for causing the immediate release of the connector after said selectors have released, a time measuring device responsive to the replacement of the calling subscribers receiver to begin counting a period of time, and means effective at the end of said period to release the connector switch if the called party still has his receiver off the switchhook.

9. The combination in a telephone system of tions to said lines, a connecting circuit incoming to said switch, means for setting said switch, means for maintaining a potential on said circuit to guard the switch against seizure, a source of current connected to said incoming circuit, means for removing said guarding potential and for disconnecting said source of current from said incoming circuit while the switch remains in its-set condition, and means for replacing the guarding potential to prevent the seizure of said switch.

10. The combination in a telephone system of lines, a selective switch for establishing connections to said lines, a connecting circuit incoming to said switch, means for setting said switch, means for maintaining a potential on said circuit to guard the switch against seizure, a source of current connected to said incoming circuit,

means efiective while said switch remains in its set condition for removing said guarding potential and for disconnecting said source to prevent the flow of current in said incoming circuit in case the switch is seized while unguarded, and means for replacing the guarding potential on said incoming circuit.

11. The combination in a telephone system of lines, a connector switch, a trunk leading to said connector switch, selectors for establishing a connection over said trunk to said connector, means for setting said connector to extend. a connection to one of said lines, means in the connector for maintaining a guarding condition on said connection, a source for supplying current to the es-- tablished connection, means in the connector for connecting said source to said trunk, means effective while said connector remains in its set condition for removing the guarding condition to cause the release of said selectors and to disconnect said source from said trunk, and means for restoring said guarding condition to prevent seizure of said connector by one of said selectors.

12. In a telephone system, subscribers lines, a connector switch for making connections with said lines, a trunk incoming to said connector having terminals, means in said connector for placing a busy and a holding potential on said terminals, selector switches for establishing a connection to said trunk, means for setting said connector switch, a source for supplying current to the established connection, means in said connector for connecting said source to said incom ing trunk, means effective while said connector remains in its set condition for removing said busy potential to cause the release of said selector switches and to disconnect said source from said trunk, and means for restoring said busy potential to prevent the seizure of said connector by one of said selectors.

13. In a telephone system, a calling line and a called line, a connector switch for making a connection to the called line, a trunk circuit incoming to said connector provided with connecting terminals, selector switches for seizing said terminals to extend a connection to said connector switch, means for setting said connector switch, means in said connector switch for maintaining a busy and a holding potential on said incoming trunk terminals, a source for supplying current to the established connection, means in said connector for connecting said source to said incoming trunk, means controlled by the calling subsoriber for removing said potential to cause the release of said selector switches and for disconnecting said source of current from said trunk without releasing said connector switch, means for restoring said potential to prevent the reseizure of said connector by one of said selectors, and means controlled by the called subscriber for releasing said connector switch.

14. In a telephone system, a calling line and a called line, a connector switch for making a connection to the called line, a trunk circuit incoming to said connector provided with connecting terminals, selector switches for seizing said terminals to extend a connection to said connector switch, means for setting said connector switch, means in said connector switch for maintaining a busy and a holding potential on said incoming trunk terminals, a source for supplying current to the established connection, means in the connector switch for connecting said source to the incoming trunk, means efiective when the called subscriber answers for reversing the direction of current flow from said source, means effective while said connector remains in its set condition for removing the busy potential from said trunk to cause the release of said selectors and for disconnecting said source from said trunk, and means for restoring said busy potential to prevent reseizure of said connector by one of said selector switches.

15. The combination in a telephone system of a calling line and a called line, a connector switch, a trunk incoming thereto, selector switches for extending a connection from the calling line to said connector switch, means for operating said connector switch to establish a connection to the called line, means for maintaining a potential on said trunk circuit, a source for supplying current to the established connection, means in the connector for connecting said source to said incoming trunk, means eiTective when the called subscriber answers for reversing the direction of current flow from said source, means in the established connection responsive to such reversal of current, means effective while said connector remains in its set condition for removing said potential to cause the release of said selectors and for disconnecting said source from said trunk, and means for restoring said potential to prevent the reseizure of said connector by one of said selectors.

T6. The combination in a telephone system of a calling line and a called line, a connector switch having a trunk incoming thereto, selector switches for establishing a connection from the calling line to said connector switch, means for operating said connector to complete the connection to the called line, means in the connector switch for maintaining a. potential on said trunk, a source for supplying current to the established connection, means in the connector switch for connecting said source to said incoming trunk, means responsive to the replacement of the calling subscriber's receiver for removing said potential to cause the release of said selector switches without releasing said connector and for disconnecting said source from said trunk, means for restoring said potential to said trunk to prevent reseizure of the connector by one of said selectors, means controlled by the called subscriber for releasing said connector switch, and time controlled means fcr releasing said connector switch in case the called subscriber fails to replace his receiver on the swi'tchhook.

HENRY HOVLAND. 

